A pulse generator is one of many medical devices that are implantable in a patient and provide a therapy that is dependent on the current status of the patient. For example, a pacemaker is a widely used medical device that includes a pulse generator for providing stimulus to cardiac tissue. The amount of stimulus provided corresponds to the activity level of the patient. A patient that is sleeping requires lower stimuli than a person that is active and in motion. One method for determining the status of the patient is to use an accelerometer.
An accelerometer measures changes in a patient's physical activity. The physical changes are detected by the accelerometer and algorithmically interpreted by circuitry within the pulse generator to produce a modified therapy that is correct for the current activity level. The accelerometer is placed within the implantable medical device. One type that has been successfully implemented in a pulse generator is a single axis accelerometer that measures both dynamic and static acceleration (e.g. gravity) in a single direction. Measurement in all three dimensions is achieved by using three single axis accelerometers respectively mounted to detect in the x, y, and z axis.
FIG. 1 is an isometric exploded view of a known accelerometer 10. Accelerometer 10 comprises a substrate 20, a substrate 30, and a substrate 40. In general, substrates 20, 30, and 40 are made of silicon. A moveable mass 50 is centrally located in substrate 30. The moveable mass is coupled to the main body of substrate 30 by flexures 60. For reference, the x, y, and z directions are shown relative to substrates 20, 30, and 40. Flextures 60 allow moveable mass 50 to move in the z-direction. A lower surface of substrate 20 couples to an upper surface of substrate 30. An upper surface of substrate 40 couples to a lower surface of substrate 30. Accelerometer 10 is sealed from an external environment when substrates 20, 30, and 40 are coupled together.
Moveable mass 50 is a conductive element. An upper surface of moveable mass 50 is spaced a predetermined distance from a conductive surface on the lower surface of substrate 20 forming a first capacitor. Similarly, a lower surface of moveable mass 50 is spaced a predetermined distance from a conductive surface on the upper surface of substrate 40 forming a second capacitor. The value of both the first and second capacitor changes as the moveable mass 50 moves. In an embodiment of accelerometer 10, the values of the first and second capacitors are used differentially such that the difference in capacitor values is detected. For example, moving mass 50 moves closer to the conductive surface on the lower surface of substrate 20 increasing the value of the first capacitor. Conversely, moveable mass 50 is moving farther away from the conducting surface of the upper surface of substrate 40 decreasing the value of the second capacitor. The difference between the first and second capacitors values is detected and corresponds to the movement induced in moving mass 50.
In general, moveable mass 50 is formed from the material comprising substrate 30. A wet etch is used to separate moveable mass 50 from substrate 30. The wet etch process leaves a substantial distance between moveable mass 50 and substrate 30. Flexures 60 are designed to flex which allows movement of moveable mass 50 in the z-direction. Flexures 60 are not flexible in the y-direction and may crack or fracture under conditions of high g-force in the y-direction. For example, dropping accelerometer 10 can produce movement in the y-direction where moveable mass 50 hits a sidewall of substrate 30. The distance between moveable mass 50 and the sidewall of substrate 30 is such that sufficient movement is generated to stress flexures 60 into cracking or fracturing.
FIG. 2 is an isometric view of a prior art accelerometer 100 coupled to a substrate 160. Accelerometer 100 includes substrates 110, 120, and 130 coupled together similar to that described in FIG. 1. An end cap 140 and an end cap 150 are coupled to substrates 110, 120, and 130. End caps 140 and 150 are primarily used for providing interconnection and physically fastening to substrate 160. Substrates 110, 120, and 130 have exposed interconnect (not shown) that abuts and couples to interconnect (not shown) on end cap 150. Interconnect 180 is coupled to the exposed interconnect (not shown) on substrates 110, 120, and 130. The three separate interconnects comprising interconnect 180 correspond to the terminals of two capacitors with one terminal common to both capacitors. The common terminal is the moveable mass in accelerometer 100.
End caps 140 and 150 increase the size, add complexity, and cost to the manufacture of accelerometer 100. Interconnect 180 aligns with and couples at a right angle to interconnect 170 on substrate 160. Solder or an adhesive epoxy is used to electrically couple interconnect 180 to interconnect 170. Although not shown, interconnect 170 typically couples to other circuitry (not shown) coupled to substrate 160.
Accordingly, it is desirable to provide a more reliable accelerometer. In addition, it is desirable to provide an accelerometer that is simple to manufacture and lower cost. It would be of further benefit if the accelerometer had a small footprint and was easily coupled to a substrate. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.